1. Field of the Invention
This invention relates to a tough, wear-resistant polyurethane varnish and flooring materials coated therewith.
2. Prior Art
The need for a protective surface coating on rigid flooring materials has long been present. A coating is necessary, not only to prevent undue wear of the surface being protected, but also to seal the surface against external influences such as moisture, light and corrosive elements in the atmosphere. Such surface coating materials are commonly liquid applications solidifying soon afterwards. Early surface coating materials included oils, wax, egg albumin, and glue. The first floor varnishes were solutions of natural resins, characterized by transparency, hardness, amorphous (not crystalline) structure, and virtually no long-lasting integrity. They developed cracks on aging, even when used indoors.
Present day protective coatings are now mainly synthetic resins which are generally not subject to variation in availability or quality as were their natural predecessors. Polyurethane coatings, first developed in Germany, have found wide commercial acceptance as coating compositions. These materials possess a desirable combination of flexibility, toughness, abrasion and chemical resistance. They could be formulated as clear varnishes and cured either at room temperature or elevated temperatures. (The meaning of the term "varnish" as used herein is in the general sense as any liquid preparation or composition which may be applied to a substrate to dry to provide a protective coating.)
In the late 1950s, one-component, moisture-curable polyurethane coatings were developed. These compositions were based on reaction of water vapor with a stable isocyanate-terminated prepolymer formed of isocyanate, mainly toluene diisocyanate, and a polyfunctional polyether.
A need has developed for a high performance, highly abrasion-resistant and chemically-resistant, transparent, substantially non-yellowing, polyurethane composition which may be applied to elastomeric floor covering materials such as those having a decorative surface which may be textured. A floor covering material of this type is disclosed in assignee's U.S. Pat. No. 4,103,056 as being a resilient wood replication formed of a thick elastomeric polyurethane base having a wood-stained molded textured wood-grain surface which is overcoated with a clear, tough, abrasion-resistant, flexible, water-resistant polyurethane protective coating.
The protective coating of such a floor covering material must be sufficiently resilient to retain its integrity even when stressed, for example, by the imposition of a narrow high heel of a woman's shoe or the end of a small cross sectional area leg of a heavy piece of furniture. Additionally, the coating composition must also provide sufficient wear resistance and chemical resistance to provide long-term use under pedestrian and other traffic without failure. Also important is the requirement that the protective coating remain substantially clear, without yellowing, particularly where the floor covering material is light-colored.